The present invention relates to a paper tractor for use in serial printers or the like devices.
Generally, in the known paper tractors, a sprocket wheel acting as a driving wheel is rotationally driven by a drive shaft having a polygonal cross-section or by a spline shaft. The sprocket wheel is movable in the axial direction so that its position may be adjusted to meet various paper widths.
In the conventional paper tractors, the arrangement is such that the position of the sprocket wheel is determined by using a stationary shaft (guide shaft) in addition to the drive shaft as a reference of position, due to restrictions in the structure and assembling and from the view point of easiness of the position adjusting operation. Namely, conventional paper tractors have a member which is slidable in the axial direction of the stationary shaft. This member is adjustably fixed to the stationary shaft such that the position of the sprocket wheel in relation to the drive shaft in the axial direction of the latter is determined through the medium of this member.
The use of the stationary shaft as a reference, however, inevitably generates a clearance error between the abovementioned member and the sprocket wheel, resulting in a deterioration of the positional precision of the sprocket wheel to cause a breadthwise offset of the recording paper and degradation of the quality of printing.
The paper tractors of this kind employ impractically large number of parts and require a number of steps in the assembling process, resulting in an extremely high cost. For the inspection and maintenance of the printing head, it is necessary to demount the paper tractor. The demounting and mounting of the paper tractor require troublesome work.
Under these circumstances, the present invention aims at providing a paper tractor of a simplified construction, which is easy to assemble and which permits an easy setting of the position of the sprocket wheel, thereby overcoming the above-described problems of the prior art.